1. Field of the Invention
This invention relates to an image signal conversion device for forming a high resolution image signal by increasing the number of horizontal scanning line signals of such an image signal as a TV signal that consists of a plurality of horizontal scanning line signals.
2. Description of the Related Art
A non-interlacing method of increasing, for example, by two times the number of scanning lines of one field has been proposed for obtaining an image signal of a high degree of definition from a video signal which is obtained by the interlacing method presently employed in general, such as in the NTSC color system.
The attempt to double the number of scanning lines encounters a problem as to how to create an interpolating signal in between one scanning line and another. In respect of this, for example, a method of selecting inner-field interpolation whereby an average value of two adjacent lines within one and the same field is used as the interpolating signal or inter-field interpolation whereby a corresponding signal within another field is used as the interpolating signal as it is according to the movement of the image. It is important for this method to accurately detect the movement of the image of an object as the picture quality thus obtained depends on the detection accuracy. Generally, a discrimination between a movement or no movement is made according to occurrence or nonoccurrence of any significant difference in a signal showing a difference between a present field and a preceding or ensuing field (representing a frame difference or a difference between frames). However, in accordance with this method, there arises no significant difference on some occasion in the difference signal between the present field and the preceding or ensuing field while the present field has a move. On that occasion, there arises conspicuous deterioration in picture quality such as a double image or the like.
FIGS. 1 and 2 of the accompanying drawings show such picture quality deterioration. In FIG. 1, positions (1) to (6) respectively correspond to positions (1) to (6) shown in FIG. 2. In both the drawings FIGS. 1 and 2, hatched parts indicate selection of the inner-field interpolation. Let us assume that an object image which is shown with a thick line and is at rest in a position
(2) within a field i-1 comes to move from an area (iii) to an area (i) within another field i and then comes to a stop in a position (4) within a field i+1. In this instance, if movement detection is performed by the medium of a difference signal between the present field and a preceding or ensuing field, at the areas (i) and (iii) of the field i of FIG. 1, there arises a significant difference in the frame difference of the areas (i) and (iii) shown in FIG. 2. Therefore, the inner-field Meanwhile, interpolation is selected in this instance. There arises no significant difference in the frame difference of an area (ii). Therefore, although this area has the movement, the inter-field interpolation is selected for the area. Then, the area (ii) of FIG. 1 is interpolated with a signal representing the background image of the preceding field. As a result of this, there arises a disturbance by a double image.
Further, it is only the field i that has a move. Meanwhile, the fields i-1 and i+1 are at rest. Under the condition, inter-field interpolation is applicable. However, in accordance with the method of detecting movement through the frame difference, it is impossible to determine a field having occurrence of a move. Therefore, the inner-field interpolation is selected for the fields i-1 and i+1. This results in an unnatural (blurred) picture of a moving object having low resolution images before and after it.